The permeability of porous rock is an indication of the rate at which fluid will flow through the rock. Laboratory measurements of single-phase, steady-state permeability of porous rock are important for a number of different applications. The oil and gas industry uses permeability data as a key indicator of the ability of a hydrocarbon reservoir to be productive. In addition, the effective containment of large volumes of oil in underground salt caverns is directly dependent upon the permeability of the adjacent cavern walls. In addition, safe, long term underground isolation of radioactive and hazardous waste is contingent upon the flow and transport characteristics of the surrounding geologic formations.
There are two types of permeability tests that are typically performed today: the steady-state test and the pulse test. The steady-state test consists of measuring the flow rate and pressure drop across a rock sample under conditions of steady pressure. Steady-state techniques can be used for all rocks under all conditions, but this test is difficult and time consuming, especially for low permeability rocks, because steady conditions must be achieved and the flow rate must be measured. The pulse test is performed by subjecting a rock to an average pore pressure, opening a reservoir of known volume at a higher (or lower) pressure, and measuring the change in pressure as fluid flows into (or out of) the rock sample. This test requires no flow measurement and works well for higher permeability rocks where the permeability is it, sensitive to pressure and small pressure changes (between the reservoir and the initial pore pressure) can be used. However, for rocks that exhibit significant stress sensitivity (low-permeability and naturally fractured samples), the pulse technique may be less accurate because the permeability within the sample may be changing, and this is not accounted for by the analysis. For this and other reasons, there are many applications where steady-state measurements are preferred to pulse measurements.
The equipment for single-phase, steady-state measurements typically includes a bank of mass flow meters, a differential pressure transducer, and a back pressure regulator valve. This arrangement uses an inert gas such as nitrogen for the fluid media pumped through the sample because the inert gas will not react with constituents of the rock sample and change the rock's flow properties. Difficulties arise with this arrangement because high differential pressures and low flow rates are required when testing lower permeability rocks. Flow rates tend to approach the lower limit of the commercially available flow meters, requiring that the driving pressure, or .DELTA.p, be raised even higher so that a measurable level of flow can be maintained through the sample. However, as the pressure gradient across the sample increases, the stress gradient within the sample increases, resulting in significant deviations from in situ conditions.
A second problem with this method concerns the associated costs involved with purchasing and maintaining the mass flow meters. To cover the permeability ranges of interest, a typical permeameter incorporates three to five flow meters, with each flow meter costing up to $2000. Also, such flow meters require frequent and expensive calibration to maintain reliable accuracy, adding a periodic cost to conducting the measurements. Finally, these frequent calibrations can lead to significant down time for the permeameter system, further impeding cost effective and timely results.
Other examples of prior art methods of measuring permeability of porous rock are described in U.S. Pat. No. 4,555,934 of Garland et al. and U.S. Pat. No. 4,573,342 of Jones.